1. Field of the Invention
The present invention relates to improvements in the extraction of sugar from sliced sugar beets by diffusion, in which operation the sliced sugar beets are subjected to extraction by a countercurrent flow of hot water to produce, on the one hand, a sugar juice which is purified before being concentrated by evaporation, whereupon it is subjected to crystallization, and a pulp, on the other hand, which is pressed to extract therefrom an aqueous liquid which is recycled for the diffusion extraction and then dried.
2. Description of the Prior Art
It is known from U.S. Pat. No. 4,555,270, dated Nov. 26, 1985, to reduce the content of pectic substances in the juice and to improve the pressing of the residual pulp by a pre-treatment in which the sliced sugar beets are contacted with a calcium monosaccharate solution before the diffusion, the solution being obtained by adding quick lime or milk-of-lime to the diffusion juice. For this purpose, a layer of the sliced sugar beets is placed on a permeable conveyor belt and, during their conveyance, the sugar beets are sprinkled with a cold solution of the monosaccharate (saccharated juice) which is passed several times through the layer of sliced sugar beets to obtain a countercurrent circulation so that the concentration of the monosaccharate in the solution decreases from the first passage through the layer of sliced sugar beets (at the side of the diffusion) to the last passage (at the side where the sugar beets are charged onto the belt). Thus, the reaction of fixing the calcium on the sliced sugar beets is progressive. Before this pre-treatment, the sliced sugar beets are washed or leached with a suitable liquid to remove therefrom dry materials contained in the open cells of the sugar beets. For this purpose, purified sugar juice or, preferably, spent saccharated juice may be used, the latter having the advantage of reducing the volume of recycled purified juice.
The countercurrent circulation of the saccharated juice and the sliced sugar beets makes it necessary to sprinkle the sliced sugar beets with a solution containing a considerable amount of the monosaccharate just before they enter the diffusion stage. Despite special precautions taken (permitting the monosaccharate solution to drip out of the mass of sliced beets or pressing it out), a fraction of the monosaccharate solution is entrained with the sliced sugar beets into the diffusion apparatus. However, the calcium monosaccharate is unstable in the presence of heat and, as soon as it enters the diffusion zone, it is decomposed into saccharose and lime. The action of lime on sliced sugar beets produces a number of undesirable reactions, such as deacetylation, formation of salts of lime, floculation, etc. This disadvantage is further aggravated when the saccharated juice contains hydrated lime (CaOH.sub.2).